Did you sift your dry ingredients? It could be that you experienced what is known as "tunneling", which occurs when leavenings (baking soda and/or baking powder) find themselves in clumps, even very small ones. Rather than creating leavening evenly throughout the cake, they expand in a tunnel.

Maybe you could explain it through Physics:wormholes!Your cake turned out to be from another dimension...So good,it was out of this world!No,just kidding!!!I'm with Cynthia on this one.It's just chemical reaction.